A nitride semiconductor is one of desirable candidate direct-band-gap semiconductor materials, however, it is difficult to produce a bulk of its single crystal. Therefore, hetero-epitaxial technology is usually employed to grow GaN on a different material substrate such as sapphire, SiC, etc. by metal-organic chemical vapor deposition (MOCVD) for the present. It was shown that sapphire is a preferable substrate for growing a high efficient light-emitting device of nitride semiconductor because of its stability at high temperature under atmosphere with ammonia in an epitaxial vapor deposition process compared with the other different material substrate. When a sapphire substrate is employed, a process for forming AlGaN layer as a buffer layer on the sapphire substrate at low-temperature around 600° C. is usually employed to grow nitride semiconductor layers thereon. It can improve crystallinity of the nitride semiconductor layers.
Specifically, a nitride semiconductor element grown on a sapphire substrate is used for a blue LED, a pure-green LED with higher luminance than conventional LEDs, and an LD (laser diode). They can be applied in a full-color display; traffic lights; an image scanner; light sources such as a light source for an optical disc, which is a media, for example DVD, that is capable of memorizing a large-capacity of information; a light source for communication; a printer; etc. Further, it is anticipated that is can be applied to an electronic device such as a field-effect transistor (FET).
(Related Reference 1)
Japanese Patent Laid-Open Publication Toku-Kai No. HEI 9-129932 (1997).
However, sapphire is a low thermal conductivity insulating material. Thus, the structure of nitride semiconductor element is limited. For example, in the case of conductive substrate such as GaAs or GaP, one of electric contact portions (terminals) can be disposed on the top surface of the semiconductor device, another contact portion can be disposed on the bottom. But, both of the electric contact portions of the light-emitting element grown on the sapphire substrate should be disposed on the top surface (the same plane side). Therefore, when an insulating material such as sapphire, etc. is employed as a substrate, it reduce the effective area of light-emission compared with a conductive substrate having the same area of substrate. In addition, when an insulating substrate is employed, it reduces the number of elements (chips) that are obtained from the same diameter of a wafer.
Further, a nitride semiconductor element with an insulating substrate such as sapphire is used as face-up type or face-down type. These types have both terminals in the same plane side, so that it increases current density locally. Then, it generates heat in the element (chip), so that it accelerates deterioration of the element. In addition, wires are required for both of pn terminals in a wire-bonding process for the terminals, so that it increases chip size. Therefore it reduces yield of chips. Additionally, sapphire has high hardness and a crystal structure with hexagonal system. So that when sapphire is employed as a substrate for growth, it is requires to break into chips by scribing the sapphire substrate. Thus, it requires an additional process compared with the other substrates.
Furthermore, recently, it has been available that an LED capable of emitting in ultra-violet region is in practical use. Generally, ultra-violet region is defined as wavelength of light-emission not more than 400 nm. The band gap of GaN is 365 nm. To shorten the wavelength to not more than 365 nm, absorption of GaN of a contact layer, etc. may reduce the outgoing efficiency of the light extremely.
The present invention has been devised to solve the above problems, and therefore, is aimed at providing a highly efficient nitride semiconductor element having an opposed terminal structure, whose terminals face each other, without increasing its voltage, and a method for producing thereof. Further, it is another object to provide a high light-emitting power nitride semiconductor element even in the ultra-violet region.